This invention relates to composite elements suitable for use in the generating channels of the magnetohydrodynamic (MHD) power generator.
The generating channels mentioned above are exposed to a high-temperature, high-speed current of combustion gas plasma containing seed compounds.
The electrode walls and insulating walls respectively of an electrode material and insulating material which make up the aforementioned generating channels, therefore, are required to possess the following properties.
(a) Properties essential for both walls
Thermal resistance: The channels are classified by their working temperatures into the three types: (i) Cold channels operated at temperatures not exceeding 1000.degree. K., (ii) semi-hot channels operated at temperatures within the range of from 1500.degree. to 1800.degree. K. and (iii) hot channels operated at temperatures exceeding 1800.degree. K. Of these types, the channels of the semi-hot type are held to be advantageous from the standpoints of thermal efficiency and service life. To demonstrate their merits, they are required to offer ample thermal resistance at temperatures within the stated range.
Thermal shock resistance: Since the thermal stresses exerted upon the channels during the steady and transient states of operation are conspicuous, the channels are required to offer sufficiently high thermal shock resistance to withstand the thermal stresses.
Corrosionproofness: The channels are required to withstand corrosions caused by high-temperature reactions and electrochemical reactions with potassium, sulfur, vanadium and coal slag.
Mechanical strength at high temperatures: The channels are required to possess as high mechanical strength as possible to retain necessary resistance to fractures due to thermal stresses.
Resistance to plasma: The channels are required to withstand reactions with various kinds of charged particles contained in the high heat flux and to withstand spattering of the plasma.
Resistance to arc discharge: The material of the channels is required to withstand local oxidation, fusion or vaporization even when the electrification between the generating channels and the plasma is effected in the arc mode.
(b) Properties particularly required for electrode material
High electrical conductivity required for conduction of electrons:
High thermoelectron emissivity: If the material of the cathode is incapable of thoroughly releasing electrons, arc discharge occurs between the cathode and the plasma and degrades the electrode.
(c) Property essential for insulating wall material
Ample electric insulating property at high temperatures.
U.S. Pat. No. 4,128,776 discloses an electrode for use in the MHD generator, which comprises a metal-containing refractory ceramic matrix having a melting point of not less than 1000.degree. C. and possessing conductivity, anti-ferromagnetism and resistance to oxidation.
In the semi-hot and hot temperature regions, however, the combustion gas plasma constitutes a slightly oxidation atmosphere. In this atmosphere, the metal contained in the electrode is embrittled. Thus, the electrode cannot be advantageously used in the semi-hot or hot temperature region. Besides, metals offer less resistance to the corrosion due to the potassium compound, for example. In use of the aforementioned electrode, therefore the temperature of the combustion gas plasma has to be lowered and thereby the system per se becomes deficient in thermal efficiency and power-generating efficiency.